Meet you at the local watering hole? No use of an artificial water resource, and evidence of dehydration in hibernating bats in the prairies

While torpid, small hibernators experience negative water balance due to evaporative water loss. The use of humid hibernacula and ability to drink during periodic arousals allows most hibernators to manage this deficit over the course of a winter. Some populations of big brown bats (Eptesicus fuscus) hibernate in relatively dry rock-crevices that do not contain free standing water. We monitored the winter behaviour and physiology of one such population in the Canadian prairies. Due to the semi-arid climate, we hypothesized that these bats would experience relatively high evaporative water loss and make frequent mid-winter flights to find water. We measured serum ion concentrations and hematocrit to assess level of dehydration in bats captured during winter. We also provided a heated water tank enriched in deuterium (H-2) and used stable isotope analysis to test for elevated hydrogen isotope ratios (H-2/H-1; herein delta H-2) in the blood of bats to determine if individuals drank from the tank. We also used passive acoustic monitoring, video surveillance, and passive integrated transponder (PIT) tags to determine if bats visited the heated water tank. We found evidence of hypertonic dehydration (elevated hematocrit and concentrations of some serum ions) in bats as winter progressed. Blood delta H-2 of bats was similar to that of water on the landscape, and acoustic and video surveillance did not indicate any visits by bats to the water tank. Post-arousal dehydration is not uncommon in hibernators, which agrees with our observation that the water tank did not represent a water resource, despite it being the only open (not frozen) water available. It is unknown whether bats may exploit frozen sources of water (e.g., snow) to supplement metabolic water produced from fat catabolism.